Metrology system with projected pattern for points-from-focus type processes

ABSTRACT

A metrology system is provided including a projected pattern for points-from-focus type processes. The metrology system includes an objective lens portion, a light source, a pattern projection portion and a camera. Different lenses (e.g., objective lenses) having different magnifications and cutoff frequencies may be utilized in the system. The pattern projection portion includes a pattern component with a pattern. At least a majority of the area of the pattern includes pattern portions that are not recurring at regular intervals across the pattern (e.g., as corresponding to a diverse spectrum of spatial frequencies that result in a relatively flat power spectrum over a desired range and with which different lenses with different cutoff frequencies may be utilized). The pattern is projected on a workpiece surface (e.g., for producing contrast) and an image stack is acquired, from which focus curve data is determined that indicates 3 dimensional positions of workpiece surface points.

BACKGROUND Technical Field

This disclosure relates to precision metrology using non-contactworkpiece surface measurement (e.g., in a machine vision inspectionsystem) and, more particularly, to processes for determining Z-heightsof points on a workpiece surface.

Description of the Related Art

Precision non-contact metrology systems such as precision machine visioninspection systems (or “vision systems” for short) may be utilized toobtain precise dimensional measurements of objects and to inspectvarious other object characteristics, and may include a computer, acamera and optical system, and a precision stage that moves to allowworkpiece traversal and inspection. One exemplary prior art system isthe QUICK VISION® series of PC-based vision systems and QVPAK® softwareavailable from Mitutoyo America Corporation (MAC), located in Aurora,Ill. The features and operation of the QUICK VISION® series of visionsystems and the QVPAK® software are generally described, for example, inthe QVPAK 3D CNC Vision Measuring Machine User's Guide, publishedJanuary 2003, which is hereby incorporated by reference in its entirety.This type of system uses a microscope-type optical system and moves thestage to provide inspection images of either small or relatively largeworkpieces.

General-purpose precision machine vision inspection systems aregenerally programmable to provide automated video inspection. Suchsystems typically include GUI features and predefined image analysis“video tools” such that operation and programming can be performed by“non-expert” operators. For example, U.S. Pat. No. 6,542,180, which isincorporated herein by reference in its entirety, teaches a visionsystem that uses automated video inspection including the use of variousvideo tools.

Accuracies in the micron or sub-micron range are often desired in suchsystems. This is particularly challenging with regard to Z-heightmeasurements. Z-height measurements (along the optical axis of thecamera system) are generally derived from a “best focus” position, suchas that determined by an autofocus tool. Determining a best focusposition is a relatively complex process that generally depends oncombining and/or comparing information derived from multiple images.Thus, the level of precision and reliability achieved for Z-heightmeasurements is often less than that achieved for the X and Ymeasurement axes, where measurements are typically based on featurerelationships within a single image. Techniques that may improve orotherwise enhance the accuracy, precision and/or reliability achievedfor Z-height measurements for points on a workpiece surface would bedesirable.

BRIEF SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

A metrology system is provided including an objective lens portion, alight source, a pattern projection portion, a camera, one or moreprocessors, and a memory. The objective lens portion includes anobjective lens selected from a set of objective lenses, wherein each ofthe objective lenses in the set has a different magnification and acutoff frequency, and a magnification state of the metrology system isconfigured to be changed by changing the objective lens that is includedin the objective lens portion. The pattern projection portion includes apattern component with a pattern, wherein at least a majority of thearea of the pattern comprises a plurality of pattern portions that arenot recurring at regular intervals across the pattern, and wherein lightfrom the light source is configured to be transmitted toward the patternto form pattern light that is transmitted through the objective lens toform a projected pattern on a workpiece surface, and for which theobjective lens is configured to input image light arising from theworkpiece surface including the projected pattern and transmit the imagelight along an imaging optical path. The camera is configured to receiveimage light transmitted along the imaging optical path and provideimages of the workpiece surface including the projected pattern. Thememory is coupled to the one or more processors and stores programinstructions that when executed by the one or more processors cause theone or more processors to at least: control the light source to transmitlight toward the pattern to form the projected pattern on the workpiecesurface; utilize the camera to acquire an image stack comprising aplurality of images of the workpiece surface with the projected pattern,wherein each image of the image stack corresponds to a differentZ-height; and determine focus curve data based at least in part on ananalysis of the images of the image stack, wherein the focus curve dataindicates 3 dimensional positions of a plurality of surface points onthe workpiece surface.

According to another aspect, the plurality of pattern portions of thepattern correspond to spatial frequencies that are below the highestcutoff frequency of the objective lenses in the set of objective lenses.According to another aspect, the set of objective lenses comprises alowest magnification lens that corresponds to a lowest magnification ofthe set, a highest magnification lens that corresponds to a highestmagnification of the set, and a plurality of intermediate magnificationlenses that each correspond to a respective intermediate magnificationthat are each between the lowest and highest magnifications, and forwhich the highest magnification is at least 10 times the lowestmagnification. In various implementations, at least some of theobjective lenses of the set of objective lenses each correspond torespective magnifications of at least one of 0.5×, 1×, 2×, 2.5×, 5×,10×, 20×, 25×, 50×, or 100×.

According to another aspect, the system further includes a turret with aplurality of turret lenses, for which each turret lens corresponds to adifferent magnification, and the turret is configured to position one ofthe turret lenses in the imaging optical path. In variousimplementations, the turret may have at least first, second and thirdturret lenses, wherein the second turret lens corresponds to amagnification that is at least 2× the magnification of the first turretlens, and the third turret lens corresponds to a magnification that isat least 4× the magnification of the first turret lens, and the turretis configured to position one of the turret lenses in the imagingoptical path.

According to another aspect, each of the plurality of surface points onthe workpiece surface corresponds to a region of interest in each of theimages of the image stack and the determining of the focus curve datacomprises determining focus curve data for each of the regions ofinterest based at least in part on an analysis of the images of theimage stack, wherein for each of the surface points a peak of the focuscurve data for the corresponding region of interest indicates acorresponding Z-height of the surface point and for which the peak atleast partially results from contrast provided by pattern portions ofthe projected pattern.

According to another aspect, the pattern portions comprise darkerpattern portions and lighter pattern portions. In variousimplementations, the total amount of area of the pattern correspondingto the darker pattern portions and the total amount of area of thepattern corresponding to the lighter pattern portions are nominallyequal (e.g., at approximately a 50/50 ratio of the pattern). In variousimplementations, the lighter pattern portions correspond to spacingsbetween the darker pattern portions. In various implementations, thepattern is formed on the pattern component with a chrome on glass typeprocess.

In various implementations, the pattern portions comprise patternportions that are of different sizes. In various implementations, thedifferent sized pattern portions comprise at least first, second, thirdand fourth sized pattern portions, for which the second, third andfourth sized pattern portions each have a dimension (e.g., a length)that is at least two, three or four times, respectively, as large as acorresponding dimension of the first sized pattern portion. In variousimplementations, the different sized pattern portions further compriseat least fifth, sixth, seventh and eighth sized pattern portions, forwhich the fifth, sixth, seventh and eighth sized pattern portions eachhave a dimension that is at least five, six, seven or eight times,respectively, as large as a corresponding dimension of the first sizedpattern portion.

In various implementations, a largest pattern portion of the pluralityof pattern portions is less than twenty times the size of a smallestpattern portion of the plurality of pattern portions. In variousimplementations, a first sized pattern portion is a smallest patternportion of the plurality of pattern portions and has an area that is atleast 2 microns by 2 microns and is at most 20 microns by 20 microns. Invarious implementations, the camera comprises a pixel array for whichthe pixels each have an area that is at least at least 2 microns by 2microns and is at most 20 microns by 20 microns.

In various implementations, adjacent darker and lighter pattern portionsof the plurality of pattern portions are in sequences that are notrecurring at regular adjacent intervals across the pattern in eitherx-axis or y-axis directions of the pattern. In various implementations,the pattern may include rows and columns of pattern elements (e.g.,wherein the rows and columns may extend in the x-axis and y-axisdirections, respectively). In various implementations, each of thedarker pattern portions may consist of one or more darker patternelements (e.g., as each included in a corresponding row and column ofthe pattern). Similarly, each of the lighter pattern portions (e.g.,which may correspond to spacings between the darker pattern portions)may consist of one or more lighter pattern elements (e.g., as eachincluded in a corresponding row and column of the pattern). In variousimplementations, at least a majority of the area of the patterncomprises at least one of rows or columns of the pattern which include aplurality of pattern portions that are not recurring at regularintervals across the respective rows or columns of the pattern. Invarious implementations, for at least a majority of the rows or columnsof the pattern, all or part of each respective row or column may beunique as including a unique sequence of darker and lighter patternportions (e.g., of respective sizes) that extends across either all orpart of the respective row or column (e.g., and that is not repeated inother rows or columns of the pattern).

According to another aspect, the pattern projection portion furtherincludes a pattern positioning portion configured to be controlled toposition the pattern component in an optical path between the lightsource and the objective lens.

A method for operating a metrology system is provided. The methodincludes controlling the light source to transmit light toward thepattern included in the optical path with the objective lens to form aprojected pattern on the workpiece surface, wherein at least a majorityof the area of the projected pattern comprises a plurality of patternportions that are not recurring at regular intervals across the pattern.The camera is utilized to acquire an image stack comprising a pluralityof images of the workpiece surface with the projected pattern, whereineach image of the image stack corresponds to a different Z-height. Focuscurve data is determined based at least in part on an analysis of theimages of the image stack. The focus curve data is utilized to determine3 dimensional positions of a plurality of surface points on theworkpiece surface. In various implementations, the plurality of patternportions of the projected pattern correspond to spatial frequencies thatare below the highest cutoff frequency of the objective lenses in theset of objective lenses. According to another aspect, the method furtherincludes controlling the pattern positioning portion to position thepattern component in the optical path between the light source and theobjective lens.

In various implementations, the objective lens included in the objectivelens portion is a first objective lens that has a first cutoff frequencyand the projected pattern is a first projected pattern that is at leastpartially filtered by the first cutoff frequency and the plurality ofpattern portions are a first plurality of pattern portions, and themethod further includes changing the magnification state of themetrology system by changing the objective lens that is included in theobjective lens portion to be a second objective lens that has a secondcutoff frequency that is different than the first cutoff frequency. Thelight source is controlled to transmit light toward the pattern includedin the optical path with the second objective lens to form a secondprojected pattern on the workpiece surface, wherein the second projectedpattern is at least partially filtered by the second cutoff frequencyand at least a majority of the area of the second projected patterncomprises a second plurality of pattern portions that are not recurringat regular intervals across the pattern, and for which the secondplurality of pattern portions is different than the first plurality ofpattern portions due at least in part to the different filtering by thefirst and second cutoff frequencies. The camera is utilized to acquire asecond image stack comprising a second plurality of images of theworkpiece surface with the second projected pattern, wherein each imageof the second image stack corresponds to a different Z-height. Secondfocus curve data is determined based at least in part on an analysis ofthe images of the second image stack. The second focus curve data isutilized to determine 3 dimensional positions of a plurality of surfacepoints on the workpiece surface.

In various implementations, a pattern projection portion is provided foruse with a metrology system and includes a pattern component and apattern positioning portion. The pattern component includes a pattern,wherein at least a majority of the area of the pattern comprises aplurality of pattern portions that are not recurring at regularintervals across the pattern. The pattern positioning portion isconfigured to position the pattern component in an optical path betweenthe light source and the objective lens. After the pattern component ispositioned in the optical path, light from the light source isconfigured to be transmitted toward the pattern to form pattern lightthat is transmitted through the objective lens to form a projectedpattern on the workpiece surface, and for which the objective lens isconfigured to input image light arising from the workpiece surfaceincluding the projected pattern and transmit the image light along theimaging optical path and for which the camera is configured to receivethe image light transmitted along the imaging optical path and capturean image stack comprising a plurality of images of the workpiece surfacewith the projected pattern, wherein each image of the image stackcorresponds to a different Z-height and for which the images areconfigured to be analyzed to determine focus curve data that indicates 3dimensional positions of a plurality of surface points on the workpiecesurface. In various implementations, the plurality of pattern portionsof the pattern correspond to spatial frequencies that are below thehighest cutoff frequency of the objective lenses in the set of objectivelenses.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagram showing various typical components of ageneral-purpose precision machine vision inspection system;

FIG. 2 is a block diagram of a control system portion and a visioncomponents portion of a machine vision inspection system similar to thatof FIG. 1 and including a pattern projection portion;

FIG. 3 is a block diagram including one exemplary implementation of thepattern projection portion shown in FIG. 2 ;

FIGS. 4A and 4B are diagrams illustrating contrast focus curves as mayresult from characteristics of a projected pattern in correspondingregions of interest;

FIG. 5 is a diagram illustrating a checkerboard pattern andcorresponding power spectrum graphs;

FIG. 6 is a diagram illustrating a section of a three level fractalpattern and corresponding power spectrum graphs;

FIG. 7 is a diagram illustrating various principles for pattern portionscorresponding to different spatial wavelengths;

FIG. 8 is an enlarged section of a pattern including pattern portionsthat are not recurring at regular intervals across the pattern;

FIG. 9 is a diagram illustrating a full pattern including the patternsection of FIG. 8 with pattern portions that are not recurring atregular intervals across the pattern and corresponding power spectrumgraphs; and

FIG. 10 is a flow diagram showing a method for operating a metrologysystem including a pattern projection portion in accordance withprinciples disclosed herein.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of one exemplary machine vision inspectionsystem 10 usable as a metrology system and/or an imaging system inaccordance with principles described herein. The machine visioninspection system 10 includes a vision measuring machine 12 that isoperably connected to exchange data and control signals with acontrolling computer system 14. The controlling computer system 14 isfurther operably connected to exchange data and control signals with amonitor or display 16, a printer 18, a joystick 22, a keyboard 24, and amouse 26. The monitor or display 16 may display a user interfacesuitable for controlling and/or programming the operations of themachine vision inspection system 10. It will be appreciated that invarious implementations, a touchscreen tablet or other computingelements or the like may be substituted for and/or redundantly providethe functions of any or all of the elements 14, 16, 22, 24 and 26.

Those skilled in the art will appreciate that the controlling computersystem 14 may generally be implemented using any suitable computingsystem or device, including distributed or networked computingenvironments, and the like. Such computing systems or devices mayinclude one or more general-purpose or special-purpose processors (e.g.,non-custom or custom devices) that execute software to perform thefunctions described herein. Software may be stored in memory, such asrandom-access memory (RAM), read-only memory (ROM), flash memory, or thelike, or a combination of such components. Software may also be storedin one or more storage devices, such as optical-based disks, flashmemory devices, or any other type of non-volatile storage medium forstoring data. Software may include one or more program modules thatinclude routines, programs, objects, components, data structures, and soon that perform particular tasks or implement particular abstract datatypes. In distributed computing environments, the functionality of theprogram modules may be combined or distributed across multiple computingsystems or devices and accessed via service calls, either in a wired orwireless configuration.

The vision measuring machine 12 includes a moveable workpiece stage 32and an optical imaging system 34 that may include a zoom lens orinterchangeable objective lenses. The zoom lens or interchangeableobjective lenses generally provide various magnifications for the imagesprovided by the optical imaging system 34. Various implementations ofthe machine vision inspection system 10 are also described in U.S. Pat.Nos. 7,454,053; 7,324,682; 8,111,905; and 8,111,938, each of which ishereby incorporated herein by reference in its entirety.

FIG. 2 is a block diagram of a control system portion 120 and a visioncomponents portion 200 of a machine vision inspection system 100 (i.e.,a type of metrology system) similar to the machine vision inspectionsystem of FIG. 1 , including certain features disclosed herein. As willbe described in more detail below, the control system portion 120 isutilized to control the vision components portion 200 and a patternprojection portion 300. The control system portion 120 may be arrangedto exchange data and control signals with both the vision componentsportion 200 and the pattern projection portion 300. The visioncomponents portion 200 includes an optical assembly portion 205, lightsources 220, 230, 240, and a workpiece stage 210 having a centraltransparent portion 212. The workpiece stage 210 is controllably movablealong x- and y-axes that lie in a plane that is generally parallel tothe surface of the stage where a workpiece 20 may be positioned.

The optical assembly portion 205 includes a camera system 260 and aninterchangeable objective lens 250 included in an objective lens portion245. In some implementations, the optical assembly portion 205 mayoptionally include a variable focal length (VFL) lens 270, e.g., atunable acoustic gradient (TAG) such as that disclosed in U.S. Pat. No.9,143,674, which is hereby incorporated herein by reference in itsentirety.

In various implementations, the optical assembly portion 205 may furtherinclude a turret lens assembly 280 having lenses 286 and 288. As analternative to the turret lens assembly, in various implementations afixed or manually interchangeable magnification-altering lens, or a zoomlens configuration, or the like, may be included. In variousimplementations, the interchangeable objective lens 250 in the objectivelens portion 245 may be selected from a set of fixed magnificationobjective lenses that are included as part of a variable magnificationlens portion (e.g., a set of objective lenses corresponding tomagnifications such as 0.5×, 1×, 2× or 2.5×, 5×, 10×, 20× or 25×, 50×,100×, etc. and for which each objective lens has a cutoff frequency).

The optical assembly portion 205 is controllably movable along a z-axisthat is generally orthogonal to the x- and y-axes by using acontrollable motor 294 that drives an actuator to move the opticalassembly portion 205 along the z-axis to change the focus of the imageof the workpiece 20. The controllable motor 294 is connected to aninput/output interface 130 via a signal line 296, as will be describedin more detail below, to change the focus of the image (e.g., to changethe focus position of the objective lens 250 relative to the workpiece20). The workpiece 20 may be located on the workpiece stage 210. Theworkpiece stage 210 may be controlled to move relative to the opticalassembly portion 205, such that the field of view of the interchangeableobjective lens 250 moves between locations on the workpiece 20, and/oramong a plurality of workpieces 20.

One or more of a stage light source 220, a coaxial light source 230, anda surface light source 240 (e.g., a ring light) may emit source light222, 232, and/or 242, respectively, to illuminate the workpiece orworkpieces 20. For example, during an image exposure, the coaxial lightsource 230 may emit source light 232 along a path including a beamsplitter 290 (e.g., a partial mirror). The source light 232 is reflectedor transmitted from the workpiece 20 as image light 255, and the imagelight used for imaging passes through the interchangeable objective lens250 and the turret lens assembly 280 and is gathered by the camerasystem 260. A workpiece image exposure which includes the image of theworkpiece(s) 20, is captured by the camera system 260 (e.g., including apixel array), and is output on a signal line 262 to the control systemportion 120.

Various light sources (e.g., the light sources 220, 230, 240) may beconnected to a lighting control interface 133 of the control systemportion 120 through associated signal lines (e.g., the busses 221, 231,241, respectively). The control system portion 120 may control theturret lens assembly 280 to rotate along axis 284 to select a turretlens (e.g., having a magnification of 1×, 2×, 4×, or 6×, etc.) through asignal line or bus 281 to alter an image magnification.

As shown in FIG. 2 , in various exemplary implementations, the controlsystem portion 120 includes a controller 125, the input/output interface130, a memory 140, a workpiece program generator and executor 170, and apower supply portion 190. Each of these components, as well as theadditional components described below, may be interconnected by one ormore data/control busses and/or application programming interfaces, orby direct connections between the various elements. The input/outputinterface 130 includes an imaging control interface 131, a motioncontrol interface 132, a lighting control interface 133, and the lenscontrol interface 134.

The lighting control interface 133 may include lighting control elements133 a-133 n, that control, for example, the selection, power, and on/offswitch for the various corresponding light sources of the machine visioninspection system 100. The lighting control interface 133 also includesa pattern projection control portion 133 pp that, in the illustratedimplementation, works in conjunction with the pattern projection portion300 to provide a projected pattern during image acquisitions asdescribed in greater detail below. Briefly, the pattern projectionportion 300 is controllable to move a pattern into or out of the path ofthe source light 232. If the pattern is not positioned in the path ofthe source light 232, the source light 232 travels as described abovefor reflecting from the surface of the workpiece 20 as image light 255.If the pattern is positioned in the path of the source light 232, thenthe source light 232 is transmitted or reflected from the pattern tobecome pattern light 232′ which is reflected by the beamsplitter 290 tobe directed through the objective lens 250 to form the projected patternon the surface of the workpiece 20. The reflected image light 255′(i.e., including the projected pattern) from the surface of theworkpiece 20 passes through the interchangeable objective lens 250 andthe turret lens assembly 280 and is gathered by the camera system 260. Aworkpiece image exposure which includes the image of the workpiece(s) 20with the projected pattern, is captured by the camera system 260 (e.g.,including a pixel array of the camera system 260), and is output on asignal line 262 to the control system portion 120, and is furtherprocessed as will be described in more detail below.

The memory 140 may include an image file memory portion 141, a workpieceprogram memory portion 142 that may include one or more part programs,or the like, and a video tool portion 143. The video tool portion 143includes video tool portion 143 a and other video tool portions (e.g.,143 n) that determine the GUI, image-processing operation, etc., foreach of the corresponding video tools, and a region of interest (ROI)generator 143 roi that supports automatic, semi-automatic, and/or manualoperations that define various ROIs that are operable in various videotools included in the video tool portion 143. Examples of the operationsof such video tools for locating edge features and performing otherworkpiece feature inspection operations are described in more detail incertain of the previously incorporated references, as well as in U.S.Pat. No. 7,627,162, which is hereby incorporated herein by reference inits entirety.

The video tool portion 143 also includes an autofocus video tool 143 afthat determines the GUI, image-processing operation, etc., for focusheight measurement operations. In various implementations, the autofocusvideo tool 143 af may additionally include a high-speed focus heighttool that may be utilized to measure focus heights with high speed, asdescribed in more detail in U.S. Pat. No. 9,143,674, which is herebyincorporated herein by reference in its entirety. In variousimplementations, the high-speed focus height tool may be a special modeof the autofocus video tool 143 af that may otherwise operate accordingto conventional methods for autofocus video tools, or the operations ofthe autofocus video tool 143 af may only include those of the high-speedfocus height tool. High-speed autofocus and/or focus positiondetermination for an image region or regions of interest may be based onanalyzing the image to determine a corresponding quantitative contrastmetric for various regions, according to known methods. For example,such methods are disclosed in U.S. Pat. Nos. 8,111,905; 7,570,795; and7,030,351, each of which is hereby incorporated herein by reference inits entirety.

In the context of this disclosure, and as is known by one of ordinaryskill in the art, the term “video tool” generally refers to a relativelycomplex set of automatic or programmed operations that a machine visionuser can implement through a relatively simple user interface. Forexample, a video tool may include a complex pre-programmed set ofimage-processing operations and computations that are applied andcustomized in a particular instance by adjusting a few variables orparameters that govern the operations and computations. In addition tothe underlying operations and computations, the video tool comprises theuser interface that allows the user to adjust those parameters for aparticular instance of the video tool. It should be noted that thevisible user interface features are sometimes referred to as the videotool, with the underlying operations being included implicitly.

One or more display devices 136 (e.g., the display 16 of FIG. 1 ) andone or more input devices 138 (e.g., the joystick 22, keyboard 24, andmouse 26 of FIG. 1 ) may be connected to the input/output interface 130.The display devices 136 and input devices 138 may be used to display auser interface that may include various graphical user interface (GUI)features that are usable to perform inspection operations, and/or tocreate and/or modify part programs, to view the images captured by thecamera system 260, and/or to directly control the vision componentsportion 200.

In various exemplary implementations, when a user utilizes the machinevision inspection system 100 to create a part program for the workpiece20, the user generates part program instructions by operating themachine vision inspection system 100 in a learn mode to provide adesired image-acquisition training sequence. For example, a trainingsequence may comprise positioning a particular workpiece feature of arepresentative workpiece in the field of view (FOV), setting lightlevels, focusing or autofocusing, acquiring an image, and providing aninspection training sequence applied to the image (e.g., using aninstance of one of the video tools on that workpiece feature). The learnmode operates such that the sequence(s) are captured or recorded andconverted to corresponding part program instructions. Theseinstructions, when the part program is executed, will cause the machinevision inspection system to reproduce the trained image acquisition andcause inspection operations to automatically inspect that particularworkpiece feature (that is the corresponding feature in thecorresponding location) on a run mode workpiece, or workpieces, whichmatches the representative workpiece used when creating the partprogram. In some implementations, such techniques may be utilized tocreate a part program instruction for analyzing a reference objectimage, to provide functions and operations described in more detailbelow.

The video tool portion 143 also includes Z-height measurement toolsportion 143 z, which provides various operations and features related toZ-height measurement operations, as described in greater detail below.In one implementation, the Z-height measurement tools portion 143 z mayinclude Z-height tools 143 zt. The Z-height tools 143 zt may include anautofocus tool 143 af, and a multipoint autofocus tool 143 maf, forexample. The Z-height tools 143 zt may govern certain aspects of imagestack acquisition and related pattern projection operations andprocessing in conjunction with the Z-height tools that are configured ina mode that determines best focus heights and/or Z-height measurements(e.g., as part of points from focus type operations) based on techniquesdescribed further below.

Briefly, the Z-height measurement tools portion 143 z may perform atleast some operations similarly to known Z-height measurement tools, forexample, performing operations in learn mode and run mode for generatingall or parts of focus curves, and finding peaks as best focus positions,etc. Additional Z-height measurement tool operations which correspond toprinciples and operations as disclosed herein may also be performed. Forexample, in various implementations the autofocus tool 143 af and/orother Z-height tool of the Z-height tools 143 zt may be selected andutilized by a user for performing a points-from-focus (PFF) type processon a workpiece surface, including controlling the pattern projectioncontrol portion 133 pp for projecting a pattern on the workpiece surfaceand capturing an image stack for determining 3 dimensional positions(i.e., including Z-heights) of a plurality of surface points on theworkpiece surface.

FIG. 3 is a block diagram showing a portion of the vision systemcomponents portion 200 of FIG. 2 and including additional details of oneexemplary implementation of the pattern projection portion 300, inaccordance with certain principles disclosed herein. As illustrated inFIG. 3 , an objective lens portion 245 includes an objective lens 250-1selected from a set of objective lenses 248, wherein each of theobjective lenses in the set (e.g., objective lenses 250-1, 250-2, 250-3,. . . 250-n) has a different magnification and spatial resolution, or inother words, a cutoff frequency (e.g., for which the different objectivelenses 250 may each have a different cutoff frequency). A magnificationstate of the vision components portion 200 (e.g., an optical systemincluding optics) of the metrology system 100 is configured to bechanged by changing the objective lens 250 that is included in theobjective lens portion 245 (e.g., which may also change a correspondingcutoff frequency of the system in accordance with the objective lensthat is selected). In various implementations, the set 248 ofinterchangeable objective lenses 250 (e.g., fixed magnificationobjective lenses) may each correspond to different magnifications (e.g.,including at least some of 0.5×, 1×, 2×, 2.5×, 5×, 7.5×, 10×, 20×, 25×,50×, 100×, etc.)

The pattern projection portion 300 includes a pattern component 302, apattern positioning portion 330, a turning mirror 320 and a projectionlens 325. The pattern component 302 includes a pattern 305 (e.g., formedfrom a chrome on glass process or other fabrication method). As will bedescribed in more detail below with respect to FIGS. 8 and 9 , invarious implementations at least a majority (e.g., 50% or more) of thearea of the pattern 305 may comprise a plurality of pattern portionsthat are not recurring at regular intervals across the pattern. Inoperation, such configurations may provide a broad spectrum of spatialfrequencies in the power spectrum, as may be advantageous when used withdifferent objective lenses 250 of the set 248 which may each have adifferent cutoff frequency. More specifically, as will be described inmore detail below, even if some frequency content is filtered by thecutoff frequency of some of the objective lenses, some range of lowerfrequencies may still make it through those objective lenses so as to bepresent in the projected pattern and provide sufficient contrast so thataccurate/reliable focus curve data may be obtained.

The pattern positioning portion 330 includes a controllable motor 332and a set of rollers 334. The controllable motor 332 (e.g., ascontrolled by a control signal from the pattern projection controlportion 133 pp or otherwise) drives an actuator to move the patterncomponent 302 with the pattern 305 into or out of the source opticalpath SOP between the light source 230 and the objective lens 250-1. Inoperation, once the pattern positioning portion 330 has been controlledto position the pattern component 302 with the pattern 305 into thesource optical path SOP, the light source 230 is controlled to transmitlight through the pattern 305 to form the projected pattern 305′ on theworkpiece surface of the workpiece 20. More specifically, source light232 from the light source 230 is transmitted through the pattern 305 toform pattern light 232′ that is transmitted along the source opticalpath SOP which includes the turning mirror 320, the projection lens 325,the beamsplitter 290, and the objective lens 250-1. In particular, thepattern light 232′ is reflected by the turning mirror 320 and passesthrough the projection lens 325 and is reflected by the beamsplitter 290to pass through the objective lens 250-1 to form the projected pattern305′ on the surface of the workpiece 20′. The objective lens 250-1inputs image light 255′ arising from the workpiece surface including theprojected pattern 305′ and transmits the image light 255′ along animaging optical path TOP (i.e., which includes the image light 255′passing through the beamsplitter 290 and a lens of the turret 280 to thecamera 260). The camera 260 receives the image light 255′ transmittedalong the imaging optical path TOP (i.e., which in the illustratedexample also corresponds to an optical axis OA of the vision componentsportion 200 as corresponding to the optical axis of the objective lens250-1) and provides images of the surface of the workpiece 20′ includingthe projected pattern. The camera 260 includes a sensor SA (e.g.,including a pixel array as will be described in more detail below).

As will be described in more detail below with respect to FIGS. 4A and4B, in various implementations (e.g., as part of a points from focustype process) the camera 260 is utilized to acquire an image stackcomprising a plurality of images of the workpiece surface with theprojected pattern, wherein each image of the image stack corresponds toa different Z-height. Focus curve data (e.g., for which the projectedpattern contributes contrast) is determined based at least in part on ananalysis of the images of the image stack, wherein the focus curve dataindicates 3 dimensional positions (e.g., including Z-heights as well asrelative x-axis and y-axis positions) of a plurality of surface pointson the workpiece surface.

It will be appreciated that in the illustrated system the patternprojection optics include projection and imaging optics. In general, inthe vision system components portion 200 of the system 100 there are atleast three different clear apertures that fundamentally limit thefrequencies that can be imaged, including that of the projection lens325, the objective lens 250 and the lens of the turret 280 (e.g., alsoreferenced as a tube lens which may be lens 286 or 288, and/or otherlens included in the turret 280). As described herein, in certainimplementations it may be the characteristics of the different objectivelenses 250 and the corresponding cutoff frequencies that may beparticularly relevant. In some implementations, the projection lens 325may also have a cutoff frequency that is relevant, and that maysimilarly be included in the relevant system determinations (e.g., forthe minimum pattern element size, etc.) as discussed herein. It will beappreciated that in the system 100, in various implementations theturret lenses 280 may be physically located just in front of the camera260, and are in the imaging optical path IOP after the objective lens250, and may not significantly alter the frequency spectra of theimaging system, although in other implementations the lenses of theturret 280 may play a larger role and for which the characteristics(e.g., cutoff frequencies, etc.) may also be included in the relevantsystem determinations.

FIGS. 4A and 4B are diagrams illustrating contrast focus curves 401 and402 as may result from characteristics of a projected pattern incorresponding regions of interest. In accordance with principlesdisclosed herein, it is generally desirable for a projected pattern toprovide sufficient contrast so that the peak of a contrast curve can beaccurately localized and reliably distinguished from noise for alldesired regions of interest. As will be described in more detail below,focus curve data may be determined from analysis of an image stack(e.g., as part of points-from-focus (PFF) type processes/measurementoperations), which indicates 3-dimensional positions of surface pointson the surface of the workpiece.

FIGS. 4A and 4B illustrate how an image stack obtained by the system 100(e.g., including the pattern projection portion 300) may be utilized todetermine the Z-heights of points on a workpiece surface. In variousimplementations, the image stack is obtained by the system 100 operatingin a points-from-focus (PFF) mode (or similar mode), to determineZ-heights of points on the workpiece surface. The PFF image stack may beprocessed to determine or output a Z-height coordinate map (e.g. a pointcloud) that quantitatively indicates a set of 3 dimensional surfacecoordinates (e.g., corresponding to a surface shape or profile of theworkpiece).

In the PFF type analysis as described herein, each of the focus curves401 and 402 (as shown in FIG. 4A) corresponds to a single point on theworkpiece surface. That is, the peak of each focus curve indicates theZ-height of the single point along the direction of the optical axis OAof the vision components portion 200 of the system 100. In variousimplementations, the PFF type analysis repeats this process for multiplesurface points (e.g., each with a corresponding region of interest)across the workpiece surface such that an overall profile of theworkpiece surface can be determined. In general, the process may beperformed for the multiple surface points that are within a field ofview (i.e., as captured within the images of the image stack), where foreach image of the image stack, a particular ROI(i) corresponds to aparticular point on the workpiece surface (e.g., with the point at thecenter of the ROI).

FIGS. 4A and 4B are aligned relative to one another along the Z-heightaxis shown in the figures. FIG. 4A is a representative graph 400Aillustrating two examples of fit focus curves 401 and 402, and FIG. 4Bis a diagram of a variable focus image stack 400B which includes twodifferent regions of interest ROI(k), in particular ROI(1) and ROI(2),that correspond to the data points fm(1,i) and fm(2,i) corresponding tothe two different focus curves 401 and 402, respectively, of FIG. 4A.The regions of interest ROI(k) are included in an imaged surface regionof a workpiece.

Regarding the term “region of interest”, it will be appreciated thatsome “single point” autofocus tools return a single Z-heightcorresponding to an entire region of interest. However, known“multi-point” type autofocus tools may return multiple Z-heightscorresponding to individual “sub-regions of interest” (e.g. a grid ofsub-regions of interest) within a global region of interest defined bythe multi-point type autofocus tool. For example, such sub-regions ofinterest may be manually and/or automatically defined as centered oneach (or most) pixels within the global region of interest. Thus, insome cases, ROI(1) and ROI(2) may be regarded as representativesub-regions of interest within a global region of interest. However, theessential point is that a Z-height may be established for any definedautofocus region of interest, whether it is a region of interest of asingle point autofocus tool, or a sub-region of interest within a globalregion of interest defined by a multi-point autofocus tool. Thus, itwill be understood that when the term region of interest is used inrelation to establishing a Z-height, that sub-regions of interest (e.g.within a global region of interest defined by a multi-point autofocustool) may be encompassed within the meaning of that term. For simplicityof the current illustrations, the regions of interest ROI(1) and ROI(2)are shown to be relatively small (e.g. 3×3 pixels), although it will beappreciated that larger regions of interest (e.g., 7×7 pixels, etc.) maybe utilized in various implementations as part of such processes, etc.

As shown in FIG. 4B, each of the images image(1)-image(11) of the imagestack image(i) include the centrally located region of interest ROI(1)for which the determined focus metric values correspond to the focusmetric data points fm(1,i) on the focus curve 401. The region ofinterest ROI(1) is schematically indicated in FIG. 4B as including arelatively high level of contrast (e.g. in image (6)), corresponding tothe relatively greater focus metric values shown on the focus curve 401.Similarly, each of the images image(1)-image(11) of the image stackimage(i) include the peripherally located region of interest ROI(2) forwhich the determined focus metric values correspond to the focus metricdata points fm(2,i) on the focus curve 402. The region of interestROI(2) is schematically indicated in FIG. 4B as including a relativelylow level of contrast (e.g. in image(6)), corresponding to therelatively lesser focus metric values shown on the focus curve 402.

As shown in FIG. 4A, each focus metric value fm(1,i) or fm(2,i) may beregarded as sampling continuous underlying focus data 401S or 402S,respectively. It may be seen in FIG. 4A that the underlying focus data401S or 402S is relatively noisy (e.g. due to the small size of thecorresponding regions of interest). However, in the case of the focuscurve 401, due to higher contrast in the corresponding region ofinterest the focus metric values in the vicinity of the focus curve peak(e.g. near Zp401) are relatively large compared to the size of the“noise component” in the underlying focus data. In contrast, in the caseof the focus curve 402, due to low contrast in the corresponding regionof interest the focus metric values in the vicinity of the focus curvepeak (e.g. near Zp402) are relatively similar to the size of the “noisecomponent” in the underlying focus data.

In one specific example, the higher focus metric values indicated in thefocus curve 401 may be due at least in part to a section of a patternthat is projected on the surface area in the region of interest ROI(1)being “highly textured” and/or otherwise producing high contrast infocused images. In comparison, the lower focus metric values indicatedin the focus curve 402 may be due at least in part to a section of apattern that is projected or partially projected (e.g., partially orfully filtered by a cutoff frequency of a lens) on the surface area inthe region of interest ROI(2) having “little texture” and/or otherwiseproducing little contrast in focused images. In any case, it will beappreciated that because of the low “signal to noise” associated withthe lower peak of the focus curve 402, as compared to relatively highsignal to noise associated with the peak of the focus curve 401, thatthe estimated Z-height of the focus peak Zp402 of the focus curve 402 isless reliable or more uncertain than the estimated Z-height of the focuspeak Zp401 of the focus curve 401 (e.g., in some instances the data ofthe focus curve 402 may be considered so unreliable and/or uncertainthat no focus peak determination may reliably be made, as may beregarded as corresponding to a “gap” in the focus curve data for theworkpiece surface).

It will be appreciated that the contrast areas indicated in the regionof interest ROI(1) (e.g., in image(6)) may correspond to patternportions of a projected pattern on the workpiece surface, for which thepattern portions are of a desired size and arrangement so as to both (a)provide a desirable amount of contrast in the region of interest ROI(1)at the current magnification (e.g., including the magnification of thecurrent lens of the turret 280), and (b) not be filtered by the cutofffrequency of the current selected interchangeable objective lens 250 orother lens of the system. In comparison, the region of interest ROI(2)may be representative of certain issues that may arise when at leastpart of a pattern does not have such desirable characteristics. Forexample, the low level of contrast in the region of interest ROI(2) mayresult from a corresponding section of a pattern in which the patternportions are of a high spatial frequency that are filtered by the cutofffrequency of the current selected interchangeable objective lens 250,for which little or no of those pattern portions may be visible in theregion of interest ROI(2). As another potential issue, the scale/size ofthe pattern portions may be such that a single pattern portion (e.g., asingle darker pattern portion or a single lighter pattern portion) maycover the entire region of interest ROI(2) such that no contrast isavailable between the different pixels within the region of interestROI(2).

In accordance with principles disclosed herein, it is desirable toutilize a pattern with characteristics that will result in focus curvessimilar to focus curve 401 (e.g., with relatively high focus curvepeaks), and which may be effectively utilized with different objectivelenses with different cutoff frequencies, as well as at variousmagnifications (e.g., including the magnification of a turret lens,etc.). As will be described in more detail below, such characteristicsmay include the pattern having a broad range of spatial frequencycontent, so that even if some of the higher frequency content isfiltered/lost for some objective lenses, other lower spatial frequencies(i.e., as corresponding to certain larger pattern portions) may make itthrough and still be visible in the captured images of the image stack.In some implementations, it may be desirable for the power spectrum ofthe pattern to be relatively constant (e.g., relatively flat) out to thehighest cutoff frequency of the optical projection system (e.g., asdefined by the highest cutoff frequency of the objective lenses in a setof objective lenses that may be utilized). In some implementations, itmay also be desirable for there to be a relatively limited or smallernumber and/or maximum size of larger pattern portions (e.g., largerdarker and/or lighter pattern portions corresponding to lower spatialfrequencies that are below the lowest cutoff frequency of the set ofobjective lenses) since such pattern portions may occupy significantarea in the pattern and may cover significant numbers of pixels whenmagnified, etc. As will be described in more detail below, in certainimplementations a pseudo random pattern (e.g., such as blue noise, orsimilar), can appropriately create high frequency texture.

Points from focus type operations (e.g., including autofocus operations,etc.) associated with determining Z-heights for regions of interest havebeen previously outlined. Briefly summarizing in relation to FIGS. 4Aand 4B, a camera may move through a range of Z-height positions Z(i)along a z-axis (e.g., the focusing axis or optical axis OA) and capturean image(i) at each position. For each captured image(i), a focus metricfm(k,i) may be calculated based on a region or sub-region of interestROI(k) (e.g. a set of pixels) in the image and related to thecorresponding position Z(i) of the camera along the Z axis at the timethat the image was captured. This results in focus curve data (e.g. thefocus metrics fm(k,i) at the positions Z(i), which is one type of focuspeak determining data set), which may be referred to simply as a “focuscurve” or “autofocus curve”. In one embodiment, the focus metric valuesmay involve a calculation of the contrast or sharpness of the region ofinterest in the image. In various embodiments, the focus values orcurves may be normalized. Various focus metric calculation techniquesare described in detail in the incorporated references, and varioussuitable focus metric functions will also be known to one of ordinaryskill in the art.

The Z-height (e.g. Zp401 or Zp402) corresponding to the peak of thefocus curve, which corresponds to the best focus position along the Zaxis, is the Z-height for the region of interest used to determine thefocus curve. The Z-height corresponding to the peak of the focus curvemay be found by fitting a curve (e.g. the curve 401 or 402) to the focuscurve data (e.g. the data fm(1,i) or fm(2,i)) and estimating thelocation of the peak of the fitted curve. It will be appreciated thatwhile the image stack image(i) is shown for purposes of illustration asonly including eleven images, in an actual embodiment (e.g., as part ofa PFF type process or otherwise) a larger number of images (e.g. 100 or200 or more) may be utilized. Exemplary techniques for the determinationand analysis of image stacks and focus curves are taught in U.S. Pat.Nos. 6,542,180 and 8,581,162, each of which is hereby incorporatedherein by reference in its entirety.

FIG. 5 is a diagram illustrating a checkerboard pattern 500A andcorresponding power spectrum graphs 500B and 500C. As will be describedin more detail below, each pattern illustrated herein (e.g., in FIGS.5-9 ) is representative of both a pattern on a pattern component (e.g.,such as a pattern 305 formed on a pattern component 302 with a chrome onglass or other process) and all or part of a projected pattern (e.g.,such as projected pattern 305′ as achieved by directing light toward apattern component to project the pattern onto a workpiece surface andfor which corresponding images are captured by a pixel array of acamera). An enlarged section 510 of the checkerboard pattern 500A isillustrated, for which the pattern is shown to include pattern elementsA and B, for which pattern elements A are darker pattern elements andpattern elements B are lighter pattern elements. In an implementationformed by a chrome on glass or similar process, the darker patternelements A may correspond to chrome portions, while the lighter patternelements B may correspond to spacings between the darker patternelements A. The pattern elements A and B are each nominally of the samesize, each having a dimension x₁ along the x axis direction and adimension y₁ along the y axis direction. A corresponding spatialwavelength Wx in the x axis direction is thus equal to 2x₁, while aspatial wavelength Wy in the y axis direction is thus equal to 2y₁.

In various implementations, each pattern element A may be designated asa darker pattern portion, and each pattern element B may be designatedas a lighter pattern portion, for which in the example of the pattern500A each pattern portion may have a size of one pattern element. Asillustrated, the pattern portions (e.g., corresponding to the patternelements A and/or B) are recurring at regular intervals across thepattern 500A (e.g., in both the x axis and y axis directions of thepattern 500A). More specifically, the darker and/or lighter patternportions repeat (e.g., periodically) in regular intervals (e.g., atequally spaced positions) in both the x axis and y axis directionsacross the entire pattern.

As illustrated in the power spectrum graphs 500B and 500C, the spatialfrequency composition of the checkerboard pattern 500A results in only afew frequencies in the power spectrum. More specifically, the powerspectrum exhibits spikes at the primary spatial frequency (e.g., thespatial frequency Wx along the x axis direction) of the pattern elementsA and B (e.g., with a largest peak occurring at the frequency of 0.2 ascorresponding to the spatial wavelength Wx=2x₁) and at the higherharmonics. In comparison, a sinusoidal checkerboard pattern may exhibitonly a single sharp peak in the power spectrum.

As will be described in more detail below, in general the relatively fewfrequencies in the power spectrum may be undesirable in relation tovarious factors. As one such factor, different lenses that may beutilized by the system 100 (e.g., different objective lenses 250) mayhave different optical cutoff frequencies. For example, a high frequencycheckerboard pattern that is below the cutoff frequency of a particular2.5× magnification objective lens (and is thus appropriate for use withthat objective lens), may be above the cutoff frequency for a differentmagnification lens (e.g., a higher magnification objective lens) with adifferent optical cutoff frequency, in which case the entire pattern maybe filtered such that no pattern may be visible when projected with thehigher magnification objective lens.

FIG. 6 is a diagram illustrating a section of a three-level fractalpattern 600A and corresponding power spectrum graphs 600B and 600C. Anenlarged section 610 of the pattern 600A is shown to include two typesof pattern arrangements, which repeat (e.g., periodically) in regularintervals (e.g., at equally spaced positions) in both the x axis and yaxis directions across the entire pattern. As illustrated by the powerspectrum graphs 600B and 600C, the pattern 600A has a more diverse powerspectrum than the checkerboard pattern 500A, but for which only a fewfrequencies dominate the spectral range. For the reasons noted above,such characteristics may also be undesirable in certain implementations(e.g., the lowest corresponding spatial frequency of the patternarrangements shown in section 610 may still be above the cutofffrequency of some objective lenses, etc.).

FIG. 7 is a diagram illustrating various principles for pattern sectionsand portions corresponding to different spatial wavelengths and inrelation to pixel sizes. As illustrated in FIG. 7 , three checkerboardpattern sections 710, 720 and 730 are of equal sizes, but eachcorrespond to different spatial wavelengths. More specifically, thepattern portions in the pattern section 720 are two times as big asthose of the pattern section 710, while the pattern portions in thepattern section 730 are three times as big as those in the patternsection 710.

The pattern section 710 includes the checkerboard arrangement of patternelements A and B, for which the pattern elements A are darker patternelements, and the pattern elements B are lighter pattern elements. Forpurposes of illustration, the pattern section 710 may be consideredanalogous to a corresponding section of the pattern 500A of FIG. 5 . Thepattern elements A and B are of equal sizes, each having a dimension x₁along the x axis direction and a dimension y₁ along the y axisdirection. A corresponding spatial wavelength along the x axis directionis thus 2x₁, while a corresponding spatial wavelength along the y axisdirection is thus 2y₁.

In the pattern section 720 (i.e., for which the pattern portions aretwice as big as those of the pattern section 710), each pattern portionhas an x axis dimension 2x₁ and a y axis dimension 2y₁, for which thecorresponding spatial wavelength along the x axis direction is 4x₁ andthe corresponding spatial wavelength along the y axis direction is 4y₁.In the pattern section 730, (i.e., for which the pattern portions arethree times as big as those of the pattern section 710) the patternportions have dimensions along the x axis direction of 3x₁, and alongthe y axis direction of 3y₁, with corresponding spatial wavelengthsalong the x axis direction of 6x₁ and along the y axis direction of 6y₁.

In relation to potential pixel sizes, in one implementation the patternsection 710 is also representative of a portion of a pixel array, inwhich each pattern element (e.g., corresponding to the projected patternon a workpiece surface as imaged by the pixel array of the camera) maybe the same size and aligned with a corresponding pixel, for which apixel array section with 6×6 pixels (i.e., as overlaid by the patternsection 710) is illustrated. In accordance with such a pixel size, thepattern portions of the pattern section 720 may each cover the area offour pixels, while the pattern portions of the pattern section 730 mayeach cover nine pixels. These comparisons illustrate certain principlesin relationship to magnification of a projected pattern (e.g., asmagnified by a turret lens, etc.). For example, a magnification of thepattern section 710 by a 2× turret lens would result in projectedpattern portions of the size of the pattern portions of the patternsection 720 at a 1× magnification (i.e., thus each covering the area offour pixels in the above example). A 3× magnification of the patternsection 710 (e.g., by a 3× turret lens) would result in projectedpattern portions of the size of the pattern portions of the patternsection 730 at a 1× magnification (i.e., for which each pattern portionwould cover the area of nine pixels). As will be described in moredetail below, such magnifications may create certain issues in relationto desired amounts of contrast between pixels (e.g., for which a patternportion covering a large group of pixels may prevent desired contrastdeterminations for pixels in the middle of that group).

As a further illustration of certain principle related to patternelement sizes and spatial frequencies, additional respective patternsections 715, 725 and 735 are shown below each of the pattern sections710, 720 and 730. The pattern portion 715 includes a pattern element Aand a pattern element B, designated as darker and lighter patternportions A1x and B1x, respectively, each having an x axis dimension x₁and y axis dimension y₁, and a corresponding spatial wavelengthdesignated as WxA1B1 which is equal to 2x₁ (i.e., similar to the x axisspatial wavelength of the pattern section 710). The pattern section 725includes darker and lighter pattern portions A2x and B2x. Each of thepattern portions A2x and B2x has an x axis dimension 2x₁ and a y axisdimension y₁ and for which a corresponding spatial wavelength WxA2B2 isequal to 4x₁ (i.e., similar to the x axis spatial wavelength of thepattern section 720). The pattern section 735 includes darker andlighter pattern portions A3x and B3x, each having an x axis dimension3x₁ and a y axis dimension y₁ and for which an x axis spatial wavelengthWxA3B3 is equal to 6x₁ (i.e., similar to the x axis spatial wavelengthof the pattern section 730).

As noted above with respect to FIG. 5 , a checkerboard pattern withelements of a single size exhibits only a single primary spatialfrequency (and some minor harmonic frequencies) in the power spectrum,and for which if utilized in a system with an objective lens with afrequency cutoff below that of the primary spatial frequency, thenlittle or no projected pattern may be visible. A pattern section 750conceptually illustrates one approach for creating a combination ofspatial frequencies in a pattern (e.g., as including a weightedcombination of the pattern sections 715, 725 and 735). Morespecifically, the pattern section 750 includes six pattern sections 715,three pattern sections 725 and two pattern sections 735. The patternsection 750 thus correspondingly includes six spatial wavelengthsWxA1B1, three spatial wavelengths WxA2B2, and two spatial wavelengthsWxA3B3. In accordance with such a combination, pattern portions ofdifferent sizes may be adjacent to one another, for which differentcombinations of spatial wavelengths may also be designated, asillustrated at the bottom of the pattern section 750 (e.g., illustratingspatial wavelengths WxB1A2, WxB2A1, WxB1A3, WxB3A1, etc.). Suchcombinations of spatial wavelengths in a pattern may have certaindesirable characteristics (e.g., adding to the spectrum of spatialfrequencies of the pattern in the power spectrum), as will be describedin more detail below with respect to FIG. 8 .

FIG. 8 is a section of a pattern including pattern portions (e.g.,darker and lighter pattern portions) that are not recurring at regularintervals across the pattern. As will be described in more detail below,the pattern section 810 of FIG. 8 illustrates an enlarged patternsection from a pattern 900A of FIG. 9 . As illustrated in FIG. 8 ,different pattern portions of varying sizes extend in both the x axisand y axis directions. The pattern section of FIG. 8 is designated asincluding 30 columns and 26 rows (e.g., of darker and lighter patternportions), and may have some characteristics similar to those of otherpatterns described above (e.g., and may be fabricated utilizing a chromeon glass or other fabrication process, etc.). As illustrated in FIG. 8 ,the pattern section 810 (i.e., as a section of the pattern 900A of FIG.9 ) is not limited to squares of a single element, or unit, size, etc.as in the checkerboard pattern 500A of FIG. 5 , and may include patternportions of multiple sizes (e.g., including multiple pattern elements)extending in both the x axis and y axis directions.

In various implementations, an element size may be defined as minimumelement size that is utilized to form a pattern. In an example utilizinga chrome on glass process to form a pattern such as that of FIGS. 8 and9 , the minimum element size may be that of the smallest illustratedsquares (e.g., for which the darker pattern elements “A” may correspondto chrome elements). In accordance with such configurations, a maximumspatial frequency of the pattern is thus defined by 2× the minimumelement size. As will be described in more detail below, while acheckerboard pattern such as that of FIG. 5 may provide a high spatialfrequency, it does not have certain desirable characteristics (e.g., aflat power spectrum), and for which more complex patterns (e.g., pattern900A including pattern section 810) that do have such desirablecharacteristics may consist of various combinations of multiple patternelements.

As some specific examples, darker and lighter pattern portions ofdifferent sizes have been labeled in the x axis direction in row 26, andin the y axis direction in column 30. For example, row 26 is shown toinclude darker and lighter pattern portions (i.e., with the darkerpattern portions with an “A” designation and lighter pattern portionswith a “B” designation similar to the other designations described abovefor the other figures). The row 26 is shown to include adjacentcombinations of lighter and darker pattern portions of different sizes,such as a sequence of pattern portions B1x, A1x, B2x, A6x, B1x, A1x,B1x, A3x, B1x, A3x, B1x, A3x, B3x, A1x and B2x. Each of these patternportions has a height along the y axis direction of 1y (e.g.,corresponding to 1 unit) and for which the number designation indicatesthe number of units along the x axis direction (e.g., pattern portionA6x has a dimension of 6× or 6 units, along the x axis direction). Asanother example, the column 30 is shown to include a sequence of patternportions A1y, B1y, A6y, B1y, A1y, B2y, A1y, B1y, A2y, B3y, A1y, B2y, A3yand B1y, each having a length along the x axis dimension of 1× (e.g.,corresponding to one unit) and a length along the y axis ascorresponding to the numbered designation. As described above withrespect to FIG. 7 , each of these adjacent combinations of differentsized pattern portions may provide different spatial wavelengthcontributions for the spatial frequencies observed in a power spectrumfor the pattern. As also noted above, each of the darker patternportions and each of the lighter pattern portions are not recurring atregular intervals (e.g., at equally spaced intervals) across thepattern. In various implementations, the lighter pattern portions maycorrespond to spacings between the darker pattern portions (i.e., forwhich the darker pattern portions as spaced by the lighter patternportions are not recurring at regular intervals across the pattern).

As further illustrated in FIG. 8 , additional pattern portions ofadditional lengths (e.g., with a total of 8 or more different unitlengths) are also illustrated. For example, as noted above patternportions A1x and A3x are illustrated in row 26, and a pattern portionA2x is illustrated in row 3, a pattern portion A4x is illustrated in row1, a pattern portion A5x is illustrated in row 4, a pattern portion A6xis illustrated in row 26, a pattern portion A7x is illustrated in row15, and a pattern portion A8x is illustrated in row 16. As furtherexamples, as noted above pattern portions B1x, B2x and B3x areillustrated in row 26, and a pattern portion B4x is illustrated in row1, a pattern portion B5x is illustrated in row 12, a pattern portion B6xis illustrated in row 1, and a pattern portion B8x is illustrated in row10. As further examples, as noted above pattern portions A1y, A2y andA3y are illustrated in column 30, and a pattern portion A4y isillustrated in column 2, a pattern portion A5y is illustrated in column7, a pattern portion A6y is illustrated in column 1, a pattern portionA1y is illustrated in column 6, and a pattern portion A8y is illustratedin column 24. As further examples, as noted above pattern portions B1y,B2y and B3y are illustrated in column 30, and a pattern portion B4y isillustrated in column 2, a pattern portion B5y is illustrated in column4, a pattern portion B6y is illustrated in column 10, a pattern portionB7y is illustrated in column 14, and a pattern portion B9y isillustrated in column 6.

As noted above, it will be appreciated that each of these differentsized darker and lighter pattern portions, as adjacent to other lighteror darker pattern portions, respectively, may provide different spatialwavelength contributions for the spatial frequencies observed in a powerspectrum for the pattern. It will be appreciated that such a variety ofadjacent combinations may in particular be achieved throughout a patternwhich is not constrained with a requirement for the pattern portions torecur at regular intervals across the pattern (e.g., a non-periodic,pseudo-random, etc. pattern). In accordance with such characteristics,smaller or larger numbers of different sized darker and lighter patternportions may provide different densities of spatial wavelengths,depending on the desired characteristics for the resulting pattern. Asnoted above, in some implementations it may also be desirable for thereto be a relatively limited or smaller number and/or maximum size oflarger pattern portions (e.g., larger darker and/or lighter patternportions corresponding to lower spatial frequencies that are below thelowest cutoff frequency of the set of objective lenses) since suchpattern portions may occupy significant area in the pattern and maycover significant numbers of pixels when magnified, etc. Thisconsideration may also influence the desired number of differentdimensions of pattern portions included in a pattern (e.g., for which itmay be desired to have a maximum dimension and/or a maximum number ofdifferent dimensions of pattern portions such as 15, 20, 30, 40, etc.).

FIG. 9 is a diagram of a pattern 900A (i.e., including the patternsection 810 of FIG. 8 ) and corresponding power spectrum graphs 900B and900C. As described above with respect to FIG. 8 , in the pattern 900Athe different sized darker and lighter pattern portions are notrecurring at regular intervals across the pattern. The correspondingpower spectrum graphs 900B and 900C illustrate certain characteristicsthat may be advantageous for certain implementations. For example, inregard to the frequency response, the scale along the horizontal axis inthe graph 900B is noted to be in a very limited range (e.g., withamplitude between 6.16 and 6.30), and for which the power spectrum isnoted to be relatively constant/flat (e.g., for which at least amajority of the power spectrum over the desired range does not vary bymore than 20%). The pattern 900A thus has spatial frequencycharacteristics resulting in a broad spectrum of frequencies in thepower spectrum. As noted above, such broad frequency content may bedesirable for various applications in which different lenses (e.g.,different objective lenses) with different cutoff frequencies may beutilized. More specifically, when the pattern is projected, even if someof the frequency content is lost for some lenses, other lowerfrequencies present may still be projected by the system with resultingsufficient contrast in the images of the workpiece for determining focuscurves with relatively high peaks (e.g., see focus curve 401 of FIG.4A). In relation to the desired range, it may be desirable for the powerspectrum to be relatively flat out to the highest cutoff frequency ofthe optical projection system (e.g., based on the highest cutofffrequency of the different lenses, such as objective lenses etc., thatmay be utilized with the system).

In various implementations, it is also desirable to have a pattern witha dark/light ratio of approximately 50/50 (e.g., wherein the totalpercentage of area of the pattern consisting of the darker patternportions is nominally/approximately 50% and thus approximately equal tothe total percentage of area of the pattern consisting of the lighterpattern portions which is also nominally/approximately 50%). As usedherein, the term “nominally” encompasses variations of one or moreparameters that fall within acceptable tolerances (e.g., with less thana 5% variation from the stated values and/or specified configuration,etc.). In addition, in various implementations, it may be desirable tohave a pattern without repeating structure so as to avoid aliasingissues as well as self-imaging issues. In general, a pattern withcharacteristics similar to the pattern 900A (e.g., which may in someinstances be defined as a pseudo random pattern, with a white noise typepower spectrum within the desired range, or otherwise) is desirable inthat it meets such objectives and does not preferentially degrade higherfrequencies (e.g., as some instances of a pink or brown noise patternmay potentially do). In various implementations, a similar pattern withblue noise characteristics may also produce desirable results, asproviding more energy at higher frequencies. In some implementations, inaddition to the A and B type pattern elements and/or pattern portions, apattern formed in accordance with principles disclosed herein may alsoinclude pattern elements and/or pattern portions of different shades,tints, colors, etc., for which each pattern element and/or patternportion may be designated as a type of darker or lighter pattern elementand/or pattern portion. For example, in such an implementation, thedarker and lighter pattern elements and/or pattern portions may be inreference to an average (e.g., an average color, shade, etc.) of thepattern, for which there may be multiple types, shades, etc. of darkerpattern elements and/or pattern portions (i.e., that are darker than theaverage of the pattern), and/or multiple types, shades, etc. of lighterpattern elements and/or pattern portions (i.e., that are lighter thanthe average of the pattern).

In certain implementations, the pattern 900A may be made of small squarepattern elements (e.g., similar to pattern elements A and B of thepattern section 710 of FIG. 7 ). In such implementations, adjacentpattern elements A in the x or y axis directions may form darker patternportions of corresponding dimensions, while adjacent pattern elements Bin the x or y axis directions may form lighter pattern portions ofcorresponding dimensions. In various implementations, such patternportions may result from certain positioning (e.g., random positioning,etc.) of the pattern elements A and/or B. As noted above, in variousimplementations the lighter pattern portions may correspond to spacingsbetween the darker pattern portions (e.g., in an implementation formedby a chrome on glass or similar process, the darker pattern elements Amay correspond to chrome portions, while the lighter pattern elements Bmay correspond to spacings between the darker pattern elements A). Atotal size of a pattern (e.g., similar to pattern 900A) in one specificexample implementation may be approximately 9 mm×9 mm (e.g., which insome instances may be large enough to be larger than most CCD or CMOSpixel arrays of certain common camera image sensors). In such animplementation where the pattern elements are approximately 5 microns,there may be a total of approximately 3.2 million pattern elements inthe pattern (i.e., (9 mm/0.005 mm){circumflex over ( )}2=3.2 million),and for which a dark/light ratio of approximately 50/50 corresponds toapproximately 1.6 million darker pattern elements A (e.g., chromepattern elements). As noted above with respect to the highest cutofffrequency of the objective lenses, it may be desirable for the patternelements (e.g., pattern elements A and B and/or corresponding dimensionsof various pattern portions) to not be smaller than a certain dimension(e.g., as corresponding to a high spatial frequency) due to the finite(e.g., lateral) resolution of the projection system (e.g., of the system100 such as due to the highest cutoff frequency of the utilized lenses,etc.). Stated another way, if the dimensions of the pattern elements arebelow a minimum cutoff dimension, then when projected the patternelements may be averaged over the area of the finite projectionresolution and may not be visible (e.g., may become blurred out, forwhich no contrast may result). In various implementations, it may bedesirable for the minimum element size to be near to the size of theminimum projection resolution of what can be passed by the opticalsystem (e.g., in order to achieve/utilize the maximum amount of contrastpossible when the objective lens with the highest cutoff frequency isutilized). In some implementations, a somewhat smaller element size mayalso be utilized for which some amount of averaging may be acceptable,as resulting in some amount of loss of contrast.

In accordance with principles disclosed herein, for a set of objectivelenses to be utilized with a system (e.g., the system 100), the highestand lowest cutoff frequencies passed by the different objective lensesmay be calculated or otherwise determined, and an element size may beselected/determined that is appropriate (e.g., a smallest patternelement/portion size may be selected/determined based at least in parton the highest cutoff frequency, and in some implementations a largestdesired pattern portion size may be selected/determined based at leastin part on a lowest cutoff frequency). In such implementations, withrespect to the smallest pattern portion/element sizes, some amount ofcontrast may be lost for the objective lenses with the lower cutofffrequencies, but for which it has been determined experimentally thatthe remaining contrast may be sufficient for certain PFF type processes,etc.

In one specific example implementation, the cutoff frequencies for a setof objective lenses for the system may correspond to pattern elementsizes that vary from 3.3 microns to 20 microns. Accordingly, a minimumpattern element size of 3.3 microns may be selected to match the highestprojection frequency cutoff (e.g., and for which a smaller element sizemay cause a loss of contrast with little or no added benefit). Inaddition, as will be described in more detail below, a minimum elementsize near to the pixel size of the pixel array of the camera 260 (e.g.,a 5 micron pixel size, or a 10 micron pixel size, etc.) may be desiredso that the magnification by the turret lenses will cause as few gaps inthe focus curve data (e.g., as utilized for PFF type processes) aspossible. In addition, a pattern element size smaller than the pixelsize may cause a loss of contrast at a 1× turret magnification (e.g.,although for which this may be acceptable in some implementations if itis not a significant contrast loss and due to the consideration that at2× turret magnification there may be a slight increase in contrast, andas may also depend on a minimum projection wavelength, etc.).

In various implementations, the primary characteristics of the turretlenses that are considered for the selection of the pattern elementsizes is the corresponding increase in the scale of the pattern asprojected (e.g., by a 2× turret lens, by a 6× turret lens, etc.), whichin some implementations is a contributing reason for keeping the minimumpattern element dimensions relatively small. More specifically, inrelation to the turret lenses (e.g., turret lenses 286 and 288 and/orother lenses of turret 280), in various implementations themagnification of those lenses may not change the frequency of what canbe projected, but does serve to magnify the resultant pattern on thepixel array of the camera. Due to this aspect, it may be desirable forthe pattern element size to be relatively as small as possible, so thatit is not overly large when magnified by the turret lenses (e.g., forturret lenses with magnifications of 2×, 6×, etc.). For example, if apattern element/portion size is relatively large to start with and thenis further magnified by a turret lens, contrast data may not be obtainedin the interior of the projected pattern element/portion (e.g., due tothe magnified pattern element/portion covering more pixels than areutilized for calculating the contrast, such as if 3×3 or 7×7 pixels areutilized for calculating contrast and the magnified patternelement/portion covers more than that number of pixels). It will beappreciated that this may be another factor for determining the desiredminimum element size of the pattern, in addition to the highest cutofffrequency of the utilized lenses, etc.

As a specific example, if a pattern element has x and y axis dimensionsof 25 microns, and is projected through a 2× or 6× turret lens, theresulting projected pattern element sizes will have dimensions of 50microns or 150 microns along the x axis and y axis directions, which maybe significantly larger than a 10 micron square sized pixel, and forwhich gaps may result in the corresponding contrast data that isproduced. More specifically, in this example for the 25 micron patternelement size, some gaps in the contrast data may result from even the 2×turret lens and the magnified 50 micron×50 micron pattern element forwhich with a camera pixel size of approximately 10 microns, the patternelement as projected with the 2× turret lens would cover approximately25 pixels (i.e., an area of 5 pixels by 5 pixels). In general, inrelation to the area used to compute contrast by the system (e.g.,utilizing a 3×3, or 7×7, etc. pixel region of interest area forcomputing contrast), gaps in contrast data may occur depending on howmany pixels are covered by a projected pattern element or portion (i.e.,a darker or lighter pattern element or portion). In addition, asillustrated in FIG. 8 , certain combinations of pattern portions mayresult in 4×4 blocks or larger of pattern elements, which at the 2× and6× turret lens magnifications may cover a significant number of pixelsfor which gaps in the contrast data may occur.

In further regard to the turret magnification, the frequency cutoff ofthe projection optics (e.g., including the objective lens) will limitthe wavelength of the projected pattern before it reaches the turretlens, for which the turret lens will then magnify the resulting portionsof the projected/filtered pattern. As noted above with regard to thecheckerboard pattern of FIG. 5 , if the pattern is regular and includesa higher spatial frequency than the cutoff frequency of the projectionoptics, then there will be no contrast in the projected pattern to bemagnified by the turret lens (i.e., all of the pattern elements/portionswill be filtered out). If the pattern has characteristics similar tothose of pattern 900A of FIG. 9 , then depending on the cutoff frequencyof the objective lens being utilized, at least some contrast will makeit through (e.g., at least some of the larger pattern portions will notbe filtered out) and the smallest wavelength scale of this projectedpattern will be approximately the wavelength of the projection opticscutoff, which will then be magnified onto the pixel array of the cameraby the turret lens. In various implementations the contrast may then bea function of the pixel size of the camera relative to the cutoffwavelength (i.e., multiplied by the magnification of the turret lens).

With regard to the camera pixel size, if the pixel size is relativelylarge (e.g., 20 microns) and the pattern element is smaller (e.g., 4microns), then the individual pixels will average together severalpattern elements and at least some amount of contrast will be lost.However, as pixel technology continues to develop, relatively smallerpixel sizes for cameras continue to be produced (e.g., below 10 microns,5 microns, etc.) for which camera pixel sizes in some implementationsmay be considered to be relatively less of a limiting factor with regardto pattern element sizes for meeting the other desired characteristicsof the system as described above. For example, in certain practicalimplementations, a smallest pattern element size may be approximately 4microns, which may also be close to what can be easily manufactured bychrome on glass technologies, etc. which in some current implementationsmay be currently in the range of approximately 1 micron square.

In relation to pixel sizes versus pattern element sizes and the spatialfrequencies that may correspondingly be resolved, if the patternwavelength is two times the pixel size (e.g., with the pattern elementsof the same size as the pixel size) the pattern wavelength can beresolved. In frequency terms, in such an implementation the spatialfrequency of the pattern is just at the limit of what can be resolved bythe sampling frequency. Alternatively, if the pattern element size has adimension that is one-half the size of the pixel dimension, the patternwavelength may be one pixel long, for which in frequency terms, thespatial frequency of the pattern would be one-half the samplingfrequency and could not be resolved. In some implementations, patternelement sizes smaller than the pixel size may still create some amountof contrast because neighboring pixels may have differing numbers oflight and dark pattern elements covering their area. However, thesmaller the element sizes, the more the overall contrast within a pixeltends to be averaged and the less effective the pattern becomes forcreating contrast. As noted above, current pixel technology (e.g., withpixel sizes at or below 10 microns, 5 microns, etc.) may make suchconcerns less of an issue, in that in various practical implementationsthe camera pixels may be able to be as small as the desired smallestpattern element sizes.

FIG. 10 is a flow diagram showing a method 1000 for operating ametrology system including a pattern projection portion in accordancewith principles disclosed herein. At a block 1010, a light source iscontrolled to transmit light toward a pattern included in an opticalpath with an objective lens to form a projected pattern on a workpiecesurface, wherein at least a majority (e.g., 50% or more) of the area ofthe projected pattern comprises a plurality of pattern portions that arenot recurring at regular intervals across the pattern. At a block 1020,a camera is utilized to acquire an image stack comprising a plurality ofimages of the workpiece surface with the projected pattern, wherein eachimage of the image stack corresponds to a different Z-height. At a block1030, focus curve data is determined based at least in part on ananalysis of the images of the image stack. At a block 1040, the focuscurve data is utilized to determine 3 dimensional positions of aplurality of surface points on the workpiece surface.

In various implementations, the method 1000 may be repeated afterchanging the magnification state of the metrology system by changing theobjective lens that is included in the objective lens portion (e.g., tobe a second objective lens that has a second cutoff frequency that isdifferent than a cutoff frequency of the previously utilized firstobjective lens). For example, after changing to the second objectivelens, the light source may be controlled to transmit light toward thepattern included in the optical path with the second objective lens toform a second projected pattern on the workpiece surface (or a differentworkpiece surface), wherein the second projected pattern is partiallyfiltered by the second cutoff frequency and at least a majority of thearea of the second projected pattern comprises a second plurality ofpattern portions that are not recurring at regular intervals across thepattern. It will be appreciated that due to the different filtering bythe second objective lens (i.e., in accordance with the second cutofffrequency), the second plurality of pattern portions may be differentthan the first plurality of pattern portions corresponding to when thefirst objective lens was utilized (e.g., more or less smaller patternportions may have been filtered out, etc.). However, in accordance withthe pattern (e.g., the pattern 305 on the pattern component 302) havingspatial frequency characteristics resulting in a broad spectrum offrequencies in the power spectrum (i.e., in accordance with principlesdisclosed herein), both the first and second plurality of patternportions in the respective first and second projected patterns may besufficient/effective for producing a desirable level of contrast inimages of the workpiece surface (e.g., for PFF type processes, etc.)

The camera may then be utilized to acquire a second image stackcomprising a second plurality of images of the workpiece surface withthe second projected pattern, wherein each image of the second imagestack corresponds to a different Z-height. It will be appreciated thatin accordance with the different magnification of the second objectivelens, the area of workpiece surface (i.e., as part of the same workpiecesurface or a different workpiece surface) that is included in the fieldof view may be different than the area of workpiece surface included inthe field of view when the first objective lens was utilized (e.g., asecond objective lens with a higher magnification than that of a firstobjective lens may result in a magnified and correspondingly smalleramount of area of a workpiece surface in the field of view). Secondfocus curve data may then be determined based at least in part on ananalysis of the images of the second image stack, and the second focuscurve data may be utilized to determine 3 dimensional positions of aplurality of surface points on the workpiece surface (e.g., as part ofPFF type processes, etc.)

In general, a pattern with characteristics in accordance with principlessuch as those disclosed herein (e.g., similar to the characteristics ofthe pattern 900A of FIG. 9 ) may be desirable for a number of reasons.For example, as noted above it may be desirable to have a pattern with abroad range of spatial frequencies since a pattern with a single spatialfrequency cannot be matched to all of the set of objective lenses and/orother lenses that may be utilized with a system (e.g., the system 100).As illustrated by the examples of FIGS. 5 and 6 , it may be difficult tomake a pattern with a broad range of frequencies unless it hascharacteristics similar to those of the pattern 900A of FIG. 9 . Incertain implementations, for the desired range (e.g., as determined inpart by the cutoff frequencies of the objective lenses and/or otherlenses to be utilized), it may be desirable for the power spectrum(e.g., which may be characterized as being equal to the Fouriertransform of the pattern squared) to be a nominally/approximatelyconstant value within the desired range. In some implementations, thismay also correspond to an approximate definition of white noise withinthe desired range. In certain implementations, a pseudo random patternsuch blue noise, or similar, can appropriately create high frequencytexture on the object. In various implementations, a type of pseudorandom pattern (e.g., a pseudo random bit map pattern, etc.) mayapproximately meet such criteria for which the minimum pattern elementsize and/or desired range of pattern portion sizes may be determined inaccordance with criteria such as those described above.

With respect to the minimum pattern element size, as described above itmay generally be desirable for the pattern to not contain higher spatialfrequencies than a desired range (e.g., for which it may be desirablefor the minimum element size utilized for the pattern to not be smallerthan certain dimensions). More specifically, due to the highest cutofffrequency as noted above, any higher spatial frequencies of the patternwould not be projected by the optics even when the system is utilizingthe lens (e.g., an objective lens) with the highest cutoff frequency. Asa result of not being projected, such higher frequencies (e.g., ascorresponding to smaller pattern portions that would not appear in theprojected pattern) would effectively lower the available contrast of thepattern which may generally be undesirable in certain implementations.In some implementations, a somewhat smaller element size may also beutilized for which some amount of averaging may be acceptable, asresulting in some amount of loss of contrast.

While preferred implementations of the present disclosure have beenillustrated and described, numerous variations in the illustrated anddescribed arrangements of features and sequences of operations will beapparent to one skilled in the art based on this disclosure. Variousalternative forms may be used to implement the principles disclosedherein. In addition, the various implementations described above can becombined to provide further implementations. All of the U.S. patents andU.S. patent applications referred to in this specification areincorporated herein by reference, in their entirety. Aspects of theimplementations can be modified, if necessary to employ concepts of thevarious patents and applications to provide yet further implementations.

These and other changes can be made to the implementations in light ofthe above-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificimplementations disclosed in the specification and the claims, butshould be construed to include all possible implementations along withthe full scope of equivalents to which such claims are entitled.

What is claimed is:
 1. A metrology system, comprising: an objective lensportion including an objective lens selected from a set of objectivelenses, wherein each of the objective lenses in the set has a differentmagnification and a cutoff frequency, and a magnification state of themetrology system is configured to be changed by changing the objectivelens that is included in the objective lens portion; a light source; apattern projection portion comprising a pattern component with apattern, wherein at least a majority of the area of the patterncomprises a plurality of pattern portions that are not recurring atregular intervals across the pattern, wherein the pattern portionscomprise darker pattern portions and lighter pattern portions, and thepattern portions comprise pattern portions that are of different sizes,for which the different sized pattern portions comprise at least first,second, third and fourth sized pattern portions, for which the second,third and fourth sized pattern portions each have a length that is atleast two, three or four times, respectively, as large as acorresponding length of the first sized pattern portion, and whereinlight from the light source is configured to be transmitted toward thepattern to form pattern light that is transmitted through the objectivelens to form a projected pattern on a workpiece surface, and for whichthe objective lens is configured to input image light arising from theworkpiece surface including the projected pattern and transmit the imagelight along an imaging optical path; a camera configured to receiveimage light transmitted along the imaging optical path and provideimages of the workpiece surface including the projected pattern; one ormore processors; and a memory coupled to the one or more processors andstoring program instructions that when executed by the one or moreprocessors cause the one or more processors to at least: control thelight source to transmit light toward the pattern to form the projectedpattern on the workpiece surface; utilize the camera to acquire an imagestack comprising a plurality of images of the workpiece surface with theprojected pattern, wherein each image of the image stack corresponds toa different Z-height; and determine focus curve data based at least inpart on an analysis of the images of the image stack, wherein the focuscurve data indicates 3 dimensional positions of a plurality of surfacepoints on the workpiece surface.
 2. The system of claim 1, wherein theplurality of pattern portions of the pattern correspond to spatialfrequencies that are below the highest cutoff frequency of the objectivelenses in the set of objective lenses.
 3. The system of claim 1, whereinthe set of objective lenses comprises a lowest magnification lens thatcorresponds to a lowest magnification of the set, a highestmagnification lens that corresponds to a highest magnification of theset, and a plurality of intermediate magnification lenses that eachcorrespond to a respective intermediate magnification that are eachbetween the lowest and highest magnifications, and for which the highestmagnification is at least 10 times the lowest magnification.
 4. Thesystem of claim 3, wherein at least some of the objective lenses of theset of objective lenses each correspond to respective magnifications ofat least one of 0.5×, 1×, 2×, 2.5×, 5×, 10×, 20×, 25×, 50×, or 100×. 5.The system of claim 1, further comprising a turret with a plurality ofturret lenses, wherein each turret lens corresponds to a differentmagnification, and the turret is configured to position one of theturret lenses in the imaging optical path.
 6. The system of claim 1,wherein each of the plurality of surface points on the workpiece surfacecorresponds to a region of interest in each of the images of the imagestack and the determining of the focus curve data comprises determiningfocus curve data for each of the regions of interest based at least inpart on an analysis of the images of the image stack, wherein for eachof the surface points a peak of the focus curve data for thecorresponding region of interest indicates a corresponding Z-height ofthe surface point and for which the peak at least partially results fromcontrast provided by pattern portions of the projected pattern.
 7. Thesystem of claim 1, wherein the total amount of area of the patterncorresponding to the darker pattern portions has less than a 5%difference from the total amount of area of the pattern corresponding tothe lighter pattern portions.
 8. The system of claim 1, wherein thelighter pattern portions correspond to spacings between the darkerpattern portions.
 9. The system of claim 8, wherein the pattern isformed on the pattern component with a chrome on glass type process. 10.The system of claim 1, wherein the different sized pattern portionsfurther comprise at least fifth, sixth, seventh and eighth sized patternportions, for which the fifth, sixth, seventh and eighth sized patternportions each have a length that is at least five, six, seven or eighttimes, respectively, as large as a corresponding length of the firstsized pattern portion.
 11. The system of claim 1, wherein a largestpattern portion of the plurality of pattern portions is less than twentytimes the size of a smallest pattern portion of the plurality of patternportions.
 12. The system of claim 1, wherein the first sized patternportion is a smallest pattern portion of the plurality of patternportions and has an area that is at least 2 microns by 2 microns and isat most 20 microns by 20 microns.
 13. The system of claim 12, whereinthe camera comprises a pixel array for which the pixels each have anarea that is at least at least 2 microns by 2 microns and is at most 20microns by 20 microns.
 14. The system of claim 1, wherein adjacentdarker and lighter pattern portions of the plurality of pattern portionsare in sequences that are not recurring at regular adjacent intervalsacross the pattern in either x-axis or y-axis directions of the pattern.15. The system of claim 1, wherein the pattern projection portionfurther comprises a pattern positioning portion configured to becontrolled to position the pattern component in an optical path betweenthe light source and the objective lens.
 16. A method for operating ametrology system, the metrology system including: an objective lensportion including an objective lens selected from a set of objectivelenses, wherein each of the objective lenses in the set has a differentmagnification and a cutoff frequency, and a magnification state of themetrology system is configured to be changed by changing the objectivelens that is included in the objective lens portion; a light source; apattern projection portion included in an optical path with theobjective lens portion and comprising a pattern component with a patternthat is configured to be projected onto a workpiece surface; a camerathat receives image light transmitted along an imaging optical path fromthe objective lens and provides images of the workpiece surfaceincluding the projected pattern; the method comprising: controlling thelight source to transmit light toward the pattern included in theoptical path with the objective lens to form a projected pattern on theworkpiece surface, wherein at least a majority of the area of theprojected pattern comprises a plurality of pattern portions that are notrecurring at regular intervals across the pattern, wherein the patternportions comprise darker pattern portions and lighter pattern portions,and the pattern portions comprise pattern portions that are of differentsizes, for which the different sized pattern portions comprise at leastfirst, second, third and fourth sized pattern portions, for which thesecond, third and fourth sized pattern portions each have a length thatis at least two, three or four times, respectively, as large as acorresponding length of the first sized pattern portion; utilize thecamera to acquire an image stack comprising a plurality of images of theworkpiece surface with the projected pattern, wherein each image of theimage stack corresponds to a different Z-height; determine focus curvedata based at least in part on an analysis of the images of the imagestack; and utilize the focus curve data to determine 3 dimensionalpositions of a plurality of surface points on the workpiece surface. 17.The method of claim 16, wherein the plurality of pattern portions of theprojected pattern correspond to spatial frequencies that are below thehighest cutoff frequency of the objective lenses in the set of objectivelenses.
 18. The method of claim 16, wherein the pattern projectionportion includes a pattern positioning portion and the method furtherincludes controlling the pattern positioning portion to position thepattern component in the optical path between the light source and theobjective lens.
 19. The method of claim 16, wherein the objective lensincluded in the objective lens portion is a first objective lens thathas a first cutoff frequency and the projected pattern is a firstprojected pattern that is at least partially filtered by the firstcutoff frequency and the plurality of pattern portions are a firstplurality of pattern portions, the method further comprising: changingthe magnification state of the metrology system by changing theobjective lens that is included in the objective lens portion to be asecond objective lens that has a second cutoff frequency that isdifferent than the first cutoff frequency; controlling the light sourceto transmit light toward the pattern included in the optical path withthe second objective lens to form a second projected pattern on theworkpiece surface, wherein the second projected pattern is at leastpartially filtered by the second cutoff frequency and at least amajority of the area of the second projected pattern comprises a secondplurality of pattern portions that are not recurring at regularintervals across the pattern, and for which the second plurality ofpattern portions is different than the first plurality of patternportions due at least in part to the different filtering by the firstand second cutoff frequencies; utilize the camera to acquire a secondimage stack comprising a second plurality of images of the workpiecesurface with the second projected pattern, wherein each image of thesecond image stack corresponds to a different Z-height; determine secondfocus curve data based at least in part on an analysis of the images ofthe second image stack; and utilize the second focus curve data todetermine 3 dimensional positions of a plurality of surface points onthe workpiece surface.
 20. A pattern projection portion for use with ametrology system, the metrology system including: an objective lensportion including an objective lens selected from a set of objectivelenses, wherein each of the objective lenses in the set has a differentmagnification and a cutoff frequency, and a magnification state of themetrology system is configured to be changed by changing the objectivelens that is included in the objective lens portion; a light source; anda camera configured to receive image light transmitted along the imagingoptical path and provide images of the workpiece surface including theprojected pattern; the pattern projection portion comprising: a patterncomponent with a pattern, wherein at least a majority of the area of thepattern comprises a plurality of pattern portions that are not recurringat regular intervals across the pattern, wherein the pattern portionscomprise darker pattern portions and lighter pattern portions, and thepattern portions comprise pattern portions that are of different sizes,for which the different sized pattern portions comprise at least first,second, third and fourth sized pattern portions, for which the second,third and fourth sized pattern portions each have a length that is atleast two, three or four times, respectively, as large as acorresponding length of the first sized pattern portion; and a patternpositioning portion configured to position the pattern component in anoptical path between the light source and the objective lens; whereinafter the pattern component is positioned in the optical path, lightfrom the light source is configured to be transmitted toward the patternto form pattern light that is transmitted through the objective lens toform a projected pattern on the workpiece surface, and for which theobjective lens is configured to input image light arising from theworkpiece surface including the projected pattern and transmit the imagelight along the imaging optical path and for which the camera isconfigured to receive the image light transmitted along the imagingoptical path and capture an image stack comprising a plurality of imagesof the workpiece surface with the projected pattern, wherein each imageof the image stack corresponds to a different Z-height and for which theimages are configured to be analyzed to determine focus curve data thatindicates 3 dimensional positions of a plurality of surface points onthe workpiece surface.
 21. The pattern projection portion of claim 20,wherein the plurality of pattern portions of the pattern correspond tospatial frequencies that are below the highest cutoff frequency of theobjective lenses in the set of objective lenses.
 22. The patternprojection portion of claim 20, wherein the different sized patternportions further comprise at least fifth, sixth, seventh and eighthsized pattern portions, for which the fifth, sixth, seventh and eighthsized pattern portions each have a length that is at least five, six,seven or eight times, respectively, as large as a corresponding lengthof the first sized pattern portion.
 23. The pattern projection portionof claim 20, wherein the first sized pattern portion is a smallestpattern portion of the plurality of pattern portions and is square. 24.The pattern projection portion of claim 23, wherein the first sizedpattern portion has an area that is at least 2 microns by 2 microns andis at most 20 microns by 20 microns.